RINT1 deficiency disrupts lipid metabolism and underlies a complex hereditary spastic paraplegia

The Rad50 interacting protein 1 (Rint1) is a key player in vesicular trafficking between the ER and Golgi apparatus. Biallelic variants in RINT1 cause infantile-onset episodic acute liver failure (ALF). Here, we describe 3 individuals from 2 unrelated families with novel biallelic RINT1 loss-of-function variants who presented with early onset spastic paraplegia, ataxia, optic nerve hypoplasia, and dysmorphic features, broadening the previously described phenotype. Our functional and lipidomic analyses provided evidence that pathogenic RINT1 variants induce defective lipid–droplet biogenesis and profound lipid abnormalities in fibroblasts and plasma that impact both neutral lipid and phospholipid metabolism, including decreased triglycerides and diglycerides, phosphatidylcholine/phosphatidylserine ratios, and inhibited Lands cycle. Further, RINT1 mutations induced intracellular ROS production and reduced ATP synthesis, affecting mitochondria with membrane depolarization, aberrant cristae ultrastructure, and increased fission. Altogether, our results highlighted the pivotal role of RINT1 in lipid metabolism and mitochondria function, with a profound effect in central nervous system development.

of age, the ophthalmologic exam revealed convergent strabismus with nystagmus, optic nerve pallor, and visual acuity 3/10. The optic nerve atrophy worsened at five years of age, with clearly abnormal visual evoked potentials and normal electroretinogram. At 6 years of age, he showed hand motor difficulties, finger-to-nose dysmetria and scissor gait with brisk deep tendon reflexes, bilateral ankle clonus and Babinski sign. He also had a worsening of sphincter control at that age. He received botulinum toxin treatment between the ages of 3 and 8 years, and was able to walk with the aid of a walker, while he used a wheelchair outdoors. Since he was 10 years old, he uses a wheelchair permanently, despite the implantation of a baclofen pump at 12 years. He also exhibited mild-moderate intellectual disability, with a severe attentional and executive disorder but a good level of language, and behavioral traits such as stubbornness, obsessiveness, and low frustration tolerance.
2 An MRI study performed at 2 years of age revealed posterior periventricular and external capsule hyperintensities in T2 and FLAIR sequences, probably corresponding to delayed myelination, and thinning of the corpus callosum. The spinal MRI was normal. At 5 years of age, MRI showed an enhancement of posterior periventricular hyperintensity and signs of cerebellar, and optic nerves and chiasm atrophy ( Figure 1C). Spectroscopy revealed increased peaks of N-acetylaspartate (NAA) and creatinine, without increased lactate.
Spinal radiographs showed large lumbar vertebrae. Metabolic studies showed a mild decrease in the plasma free carnitine level, whereas plasma lactate and CSF glucose, lactate and pyruvate levels were all normal. Brainstem auditory evoked potentials, EEG, and peripheral nerve conduction studies were normal.
At 10 years of age, he was hospitalized due to acute hepatitis with vomiting and fever.
Complementary exams performed at that moment showed AST 340 UI/L, ALT 3261 UI/L, and GGT 147 UI/L. Lactate and NH3 were normal.
Patient P2: his gestation and birth were uneventful. He showed short stature at birth (46 cm; -2.4 SD). However, his birth weight and head circumference were normal. He showed normal neurological development until he suffered fulminant hepatitis and hepatic encephalopathy (HE) in the context of an enterovirus infection when he was 1 year old.
The episode was associated with hypoglycemia, AST 7000 UI/L, prothrombin time 5%, and coagulation factor V 7%, but total bilirubin was normal. Metabolic studies showed increased ammonia (600 mol/L) and lactate (5 mmol/L) and low carnitine levels.
However, acylcarnitines, free fatty acids, urine succinylacetone and autoimmunity markers (antinuclear antibodies (ANA), smooth muscle antibodies (SMA), antibodies to liver kidney microsome type-1 (LKM1), antibodies to soluble liver antigen (SLA)/liver pancreatic antibodies (LP), antibodies to liver cytosol antigen 1 (LC1) and anti-3 mitochondrial antibodies (AMA)) were normal. The appearance of this Reye-like episode with hyperlactatemia raised the suspicion of a mitochondrial disorder. Cranial MRI, cardiac ultrasound and skeletal radiographs were normal. After recovery from that episode with normalization of transaminases, he suffered sudden death at 14 months of age. Postmortem microscopic examination revealed the presence of hepatic steatosis and vacuolization of the proximal tubules of both kidneys.
Family B: patient P3 is a female born to Spanish parents. Her gestation, birth, and family history were unremarkable. Although this patient did not show any neurodevelopmental abnormalities, she had unsteady gait with frequent falls at the age of 17 months.
Nystagmus was evidenced in association with stress or fever, and fixation and extreme gaze. At 3 years old, she suffered a general deterioration for three days in the context of a febrile illness. Physical examination revealed a wide forehead, low-set ears, convergent strabismus of the left eye, a spastic-ataxic gait with brisk tendon reflexes, ankle clonus and Babinski sign ( Figure 1B). Fundoscopic examination showed optic nerve hypoplasia.    Figure 1: the lipid molecular species detected in each class of neutral lipids in fibroblasts and plasma from patients P1 and P3 (RINT1 mut , n=2) relative to control (CTL, n=5-6) individuals ((A-B) triglyceride (TG); (C-D) diglyceride (DG); and (E-F) cholesterol ester (CE)). All data are shown as the mean ± SD. *P < 0.05, **P < 0.01, ***P < 0.001. Data in were analyzed using unpaired 2-tailed t test.